The present invention relates to a series of new derivatives of the compounds known as "milbemycins", particularly of milbemycin A.sub.3, milbemycin A.sub.4 and milbemycin D.
Milbemycin D was disclosed in U.S. Pat. No. 4,346,171, where it was referred to as "compound B-41D", and milbemycins A.sub.3 and A.sub.4 were disclosed in U.S. Pat. No. 3,950,360. These compounds may be represented by the formula (I): ##STR2## in which R.sup.1 represents a methyl group, an ethyl group or an isopropyl group, these compounds being milbemycin A.sub.3, milbemycin A.sub.4 and milbemycin D, respectively.
These milbemycin compounds may be isolated from cultures of the Streptomyces strain B-41-146, which has been deposited at the Fermentation Research Institute, Agency of Industrial Science and Technology, Ministry of International Trade and Industry, Japan, whence it is available under the accession number FERM-1438. The compounds have been found to have valuable anthelmintic and acaricidal activities.
We have now discovered a series of derivatives of these milbemycins which have demonstrated, in certain tests systems, activities, particularly against ectoparasites, better than the corresponding activities of their parent compounds.